A beaker full of death wrote:
Most episodes of the series were not about those characters. That's the nature of an episodic series.
Story over characters
Or, story over character development
Drama is about a dramatic hero. There is no story without character. The thing is, you can't destroy your hero's life every week. Consequently, most of the episodes were about the guest characters. The regulars were our entre into a world were anything was possible. It has the same principle as an anthology series.
We got to know the characters of Star Trek because we saw them in action. We got to experience things WITH them, not about them.